Solar batteries, such as photovoltaic cells and the like, are enjoying increasing popularity in a variety of applications. Such batteries are highly reliable, have a long life, and now are economically produced. Initially, the applications for such batteries were for remote electronic control systems located at substantial distances from conventional power supplies, such as weather telemetering equipment, sea lighting buoys, and the like. In the consumer field, small photovoltaic batteries are used as the power source for electronic wristwatches and portable radios.
Since photovoltaic cells generate electric power in response to illumination or irradiation by the sun's rays, it is necessary to provide a supplemental or secondary battery supply for use in conjunction with photovoltaic solar batteries when sunlight is not available for energizing the photovoltaic cell. Ideally, the photovoltaic solar battery is used to charge a rechargable secondary battery during the times that the photovoltaic battery is capable of supplying power; so that the secondary battery then can take over during the times when the photovoltaic battery is not subjected to sunlight and, therefore, is essentially inoperative. In this way, electric power may be continuously supplied to a load, irrespective of the availability of sunlight.
It is necessary to limit the charging current to the battery once the battery has been fully charged. In the past, such limitation has been effected primarily by systems which utilize a resistive shunt across the voltaic cell or power supply terminals to dissipate energy once the battery has been charged up to its fully charged level. Systems of this type require relatively large heat sinks, and rather substantial heat dissipating shunt components are necessary to dissipate all of the energy and heat which is produced by the shunt circuit. A system of this type is disclosed in the patent to Takeda et al, U.S. Pat. No. 3,979,656, issued Sept. 7, 1976. In the Takeda patent, a power transistor is used in the bypass circuit as a variable resistance shunt. As the battery charge level of the secondary battery reaches its fully charged state, increasingly large amounts of energy must be dissipated by the variable resistance shunt transistor.
Another approach to regulating the charging current supplied to a secondary battery in a solar cell/battery system is disclosed in the patent to Hartman, U.S. Pat. No. 3,384,806, issued May 21, 1968. This patent is directed to a pulse width modulated charging system using a series transistor switch between the solar cell array and the battery being charged. The ratio of the conductive time and the non-conductive time of this series switch is controlled as a function of the derivative of the output power with reference to the output current transferred to the load. The system continuously matches the output impedence of the electric power generator comprising the system with the loads this generator is supplying. The relationship of the system operation, however, to the secondary or stand-by battery, is not related to the charge condition of the battery for the embodiment shown in the circuits of this patent utilizing a solar cell array for the primary power source. In an alternative system disclosed in the Hartman patent, a thermionic generator senses the charge state of the secondary battery to vary the amount of fuel supplied to a burner which, in turn, controls the operation of a heat-to-voltage converter used to supply charging current to the battery and operating current for the system loads used.
Three other typical prior art patents utilizing solar cell/battery combinations are the patents to Ule, U.S. Pat. No. 3,696,286, issued Oct. 3, 1972; Hogrefe, U.S. Pat. No. 3,740,636, issued June 19, 1973; and, Cardwell, Jr., U.S. Pat. No. 3,816,804, issued June 11, 1974. The Ule patent discloses a battery charging circuit which has a transistor in it for periodically drawing a large current from the solar cell through an inductor under the control of a difference amplifier in order to boost the array voltage of the battery. This patent, however, does not include any system for sensing the storage battery charge condition. The Hogrefe patent is directed to a system in which a shunt amplifier loads the solar cell array through resistors in response to a preestablished charge condition of the battery. Thus, heat dissipating resistors are required, subject to the disadvantages of such systems as described above. The Cardwell patent is directed to a complex system using pulse width modulation charging of a battery and which alternately and cyclically controls the charging and discharging of the battery.
The systems of the prior art all are subject to considerable disadvantages in the manner of handling the charging of the stand-by or secondary battery when a fully charged condition is reached. During the normal operation of such systems when sunlight is available, the fully charged condition of the stand-by battery generally is attained fairly quickly, so that several hours of operation while the secondary battery is fully charged take place. It is desirable to provide a simple and effective way of controlling the charging of a secondary battery in a solar cell/battery system which does not require large heat dissipating components and which is relatively simple to implement and operate. In addition, it is desirable to provide a charging circuit where a solar power supply is utilized to effect rapid charging of a secondary battery.